Question:

I already tried several things

Purge&Reinstall gnome-terminal

Create a new user

Nothing has helped so far

Solution:1

Upgrading an operating system is a very complex procedure that involves changing thousands of components, and it's impossible to test the upgrade for every variation of hardware and software that people will use the script on. This makes the upgrade script inherently buggy. (The problem is not just with Ubuntu; I once ran a script to upgrade Windows Vista to Windows 7. The script crashed and burned, and in the end I had to install Windows 7 from scratch.) Chances are that your problem was caused by the upgrade. Here is a Ubuntu forums thread with more information and advice. I'm afraid that the fix will probably be to backup your files, wipe the partition clean, install Ubuntu from scratch, and restore the backup.

Solution:2

This seems to be a defect in ubuntu. A fresh 16.04 install has the same issues. I edited gnome-terminal.css as a workaround.

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